Sexuality and Gender 411they were gay or bisexual by their high school/college years, while the majority of men
reported knowing they were gay or bisexual by junior high/high school.
In the past several decades, a large body of research in the areas of biological differences
in the brains of heterosexual and homosexual males, genetic influences on sexual orientation,
and even prenatal influences on sexual orientation has been amassed by various scientists.
One of the earliest studies, for example, found that severe stress experienced by pregnant
women during the second trimester of pregnancy (the time during which the sexual differ-
ences in genitalia are formed) results in a significantly higher chance of any male children
becoming homosexual in orientation (Ellis et al., 1988). Another study found that homosex-
ual men and heterosexual women respond similarly (and quite differently than heterosexual
men) to a testosterone-based pheromone (glandular chemical) that is secreted in perspiration
(Savic et al., 2005). In a recent study, researchers have found that while there has yet to be any
actual gene found directly influencing the transmission of homosexuality, there are genetic
“switches” that can be passed on and that may be the reason that homosexuality tends to
run in families (Rice et al., 2012). These switches, called epi-marks, control when, where, and
how much of the information contained in our genes is expressed. There are sex-specific epi-
marks that control the sexual characteristics of the fetus during prenatal development. These
sexual characteristics include not only physical sex organ development but also sexual iden-
tity and sexual partner preference. Normally epi-marks are created anew with each genera-
tion, but occasionally they remain to be passed on to the next generation. When that happens,
they may cause the reverse of their intended effect, including a reverse of sexual orientation.
Birth order has also been the subject of research in this area, with studies suggesting
that the more older brothers a man has, the more likely the younger man is to be homosex-
ual in orientation (Blanchard, 2001; Currin et al., 2015; Kishida & Rahman, 2015; McConaghy
et al., 2006). The hypothesis is that with each male birth, the mother of these males develops
a kind of “antibody” effect against the Y chromosome, and these antibodies pass through
the placenta and affect the sexual orientation of the later-born males. Even facial structure
has been found to have some predictive value for sexual orientation (Skorska et al., 2015).
Finally, a neuroimaging study with heterosexual men and women and homosexual
men and women found that the heterosexual men and homosexual women seemed neu-
rologically similar when compared to homosexual men and heterosexual women, who
were in turn neurologically similar to each other (Savic & Lindström, 2008). A more recent
study of lateralization of the brain in homosexual men, heterosexual men, and heterosexual
women found that the homosexual men were more similar in lateralization for recogniz-
ing facial emotions to heterosexual women than they were to heterosexual men (Rahman &
Yu s u f , 2 0 1 5 ).
The evidence for genetic influences on sexual orientation is increasingly convincing.
In studies of male and female homosexual people who have identical twins, fraternal twins,
or adopted siblings, researchers found that 52 percent of the identical twin siblings were
also gay, compared to 22 percent of the fraternal twins and only 11 percent of the adopted
brothers and sisters (Bailey & Pillard, 1991). In a similar study with lesbian women only,
48 percent of identical twins were also gay compared to 16 percent of the fraternal twins
and 6 percent of the adopted siblings (Bailey et al., 1993). Other research along similar lines
has supported these findings (Bailey et al., 2000; Dawood et al., 2000:Ngun & Vilain, 2014;
Sanders et al., 2015). However, these findings should be interpreted cautiously. Twin stud-
ies are difficult to conduct without the influence of environment on behavior. Even twins
who are raised apart tend to be reared in similar environments, so that the influence of
learning and experience on sexual orientation cannot be entirely ruled out.
Some research suggests that homosexuality may be transmitted by genes carried on
the X chromosome, which is passed from mother to son but not from father to son. In 33 out
of 40 homosexual brothers, Dean Hamer and colleagues (Hamer et al., 1993) found an area
on the X chromosome (in a location called Xq28) that contains several hundred genes that
the homosexual brothers had in common in every case, even though other genes on that
chromosome were different. This was taken as evidence that the brothers had both inherited